1. Field of he Invention
The present invention relates to a dry friction material which is suitable for use as a clutch facing or a disk brake pad of automobiles, industrial machinery, railway rolling stocks and so on.
2. Description of the Related Art
A friction material requires a high and stable friction coefficient, and excellent wear resistance. It is difficult for a single raw material to satisfy these performances. So, a composite material in which many raw materials are mixed is more suitable for a friction material.
The above-mentioned friction materials are classified as follows:
(1) cork, cellulose: A few of them are single substances. Most of them are impregnated with resin and completed by thermoforming.
(2) woven: It is produced by the following manner. A string which is made of asbestos or organic fiber is impregnated with resin. The string is wound round a core which is made of glass fiber or brass wire. Then, thermoforming is performed.
(3) semi mould: It is produced by the following manner. A string is impregnated with resin. A rubber material is charged into the string. Then, thermoforming is performed.
(4) resin mould: It is produced by the following manner. A base material which is made of asbestos or glass fiber is mixed with phenol resin and many kinds of fillers. Then, thermoforming is performed.
(5) rubber mould: It is produced by the following manner. A base material which is made of asbestos or glass fiber is mixed with rubber and many kinds of fillers. Then, thermoforming is performed.
(6) semi metallic: It is produced by the following manner. A base material which is made of metal fiber is mixed with rubber and many kinds of fillers. Then, thermoforming is performed.
(7) sintered metallic: It is produced by the manner that metal powder is sintered.
(8) cermet: It is produced by the manner that ceramic powder and metal powder are sintered.
Among these, semi mould or resin mould is preferably used as a friction material of automobiles. For example, the friction material which is suitable for use as a clutch facing of automobiles is produced by the following manner. A base material which is made of glass fiber is impregnated with phenol resin and compounded rubber. Then, thermoforming is performed.
Conventionally, a dry friction material comprises fiber base material, bonding agent and friction conditioner. In the dry friction material, carbon black, graphite, cashew dust or a lead compound such as lead sulfide or lead sulfate is used as lubricant to improve wear resistance. Especially, cashew dust and the lead compound are useful to improve wear resistance when the load is low or average. However, cashew dust is effective only at the temperature in narrow ranges, and anti fade deteriorates and a friction coefficient decreases. On the contrary, the lead compound is effective at the temperature in wide ranges, and wear resistance remarkably improves. Though, usage control of the lead compound recently progresses on account of environmental contamination or pollution. So, it is necessary to substitute other materials for the lead compound.